The U.S. Supreme Court has struck down part of a program aimed at cutting pollution generated by trucks at the Los Angeles port.

Port officials can still ban dirty trucks, forcing trucking companies to switch to cleaner vehicles, as part of their 2008 clean truck program, but according to the June 13 ruling, they cannot make the companies display “how am I doing?” placards in their trucks and to provide a list of places they park vehicles that are not in service.

The justices ruled that federal law pre-empts the port from enforcing those two requirements, part of a host of “concession agreements” under the 2008 program.

Marine terminal operators face misdemeanor charges and could be fined up to $500 or sentenced to six months in prison if they let in trucks from companies that fail to comply with those agreements. Truck companies could also lose their license to operate at the port if they do not take “corrective action.”

Bill Graves, president of American Trucking Associations, the group that challenged the two requirements with the Supreme Court, hailed the decision, saying it will “send a signal to any other cities” wanting to start “similar programs which would impermissibly regulate the port trucking industry.”

ATA representatives added their goal was never to get out of phasing out older, polluting trucks.

Port representatives declined to talk specifically about what the ruling could mean for the clean truck program, which was championed by Mayor Antonio Villaraigosa.

“We are reviewing the Supreme Court ruling in order to determine how it affects our current ability to provide a clean, safe and secure trucking system consistent with the Court’s guidance,” said Phillip Sanfield, a spokesman for the Los Angeles port.

Villaraigosa, who has a week left in his term, also said he is studying the decision, but pointed to a 91 percent reduction in “harmful truck emissions” during the last five years of the truck program.

An attorney for environmental groups involved with the case said the ruling was “narrow” and pertained to two “minor contract terms.”

“The heart of the clean trucks program is intact and the port’s day-to-day operations do not need to change,” said David Pettit, a senior attorney with the Natural Resources Defense Council, which worked with the port’s legal team to defend the concession agreements on behalf of the Sierra Club and the Coalition for Clean Air.

The port had not been enforcing the two requirements struck down, he said.

Another NRDC senior attorney, Melissa Lin Perrella, said reports on where the trucking companies are parking would have addressed concerns “about dirty polluting trucks idling” on residential streets near the port, and the placards would have encouraged residents to alert the port about environmental and safety violations.

The two requirements the Supreme Court overturned were part of five concession agreements originally challenged by the truck association.

The Ninth Circuit Court of Appeals had agreed with ATA that trucking companies should be able to hire truck drivers as independent contracts rather than as employees, but struck down challenges to two other requirements that the companies show their financial ability to comply with the clean truck program and submit clean truck maintenance plans.

The port decided not to pursue keeping the driver employment requirement, which environmental and labor groups had argued was necessary because drivers operating as independent contractors were not paid enough by trucking companies to upgrade and maintain their vehicles on their own.

The Supreme Court decided the other two issues – the financial capacity and maintenance plan requirements – were not relevant.